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Micropolis: Improve decode.

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* Discard a partial sector at the end of the track.
    * Do not seek to the index mark for the first sector.
    * Use a 64-bit pattern to match the SYNC.
    * If SYNC is found too early, search for a subsequent SYNC.
    * While decoding the sector record, enforce the SYNC pattern
      and track ID.
This commit is contained in:
Howard M. Harte
2022-02-19 14:10:44 -08:00
parent 6866d5d3fa
commit a25111e411
1 changed files with 56 additions and 7 deletions
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63
arch/micropolis/decoder.cc
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@@ -8,8 +8,16 @@
#include "fmt/format.h"
#include "lib/decoders/decoders.pb.h"
/* The sector has a preamble of MFM 0x00s and uses 0xFF as a sync pattern. */
static const FluxPattern SECTOR_SYNC_PATTERN(32, 0xaaaa5555);
/* The sector has a preamble of MFM 0x00s and uses 0xFF as a sync pattern.
*
* 00 00 00 F F
* 0000 0000 0000 0000 0000 0000 0101 0101 0101 0101
* A A A A A A 5 5 5 5
*/
static const FluxPattern SECTOR_SYNC_PATTERN(64, 0xAAAAAAAAAAAA5555LL);
/* Pattern to skip past current SYNC. */
static const FluxPattern SECTOR_ADVANCE_PATTERN(64, 0xAAAAAAAAAAAAAAAALL);
/* Adds all bytes, with carry. */
uint8_t micropolisChecksum(const Bytes& bytes) {
@@ -35,24 +43,65 @@ public:
nanoseconds_t advanceToNextRecord() override
{
seekToIndexMark();
return seekToPattern(SECTOR_SYNC_PATTERN);
nanoseconds_t now = tell().ns();
/* For all but the first sector, seek to the next sector pulse.
* The first sector does not contain the sector pulse in the fluxmap.
*/
if (now != 0) {
seekToIndexMark();
now = tell().ns();
}
/* Discard a possible partial sector at the end of the track.
* This partial sector could be mistaken for a conflicted sector, if
* whatever data read happens to match the checksum of 0, which is
* rare, but has been observed on some disks.
*/
if (now > (getFluxmapDuration() - 12.5e6)) {
seekToIndexMark();
return 0;
}
nanoseconds_t clock = seekToPattern(SECTOR_SYNC_PATTERN);
auto syncDelta = tell().ns() - now;
/* Due to the weak nature of the Micropolis SYNC patern,
* it's possible to detect a false SYNC during the gap
* between the sector pulse and the write gate. If the SYNC
* is detected less than 100uS after the sector pulse, search
* for another valid SYNC.
*
* Reference: Vector Micropolis Disk Controller Board Technical
* Information Manual, pp. 1-16.
*/
if ((syncDelta > 0) && (syncDelta < 100e3)) {
seekToPattern(SECTOR_ADVANCE_PATTERN);
clock = seekToPattern(SECTOR_SYNC_PATTERN);
}
return clock;
}
void decodeSectorRecord()
{
readRawBits(16);
readRawBits(48);
auto rawbits = readRawBits(MICROPOLIS_ENCODED_SECTOR_SIZE*16);
auto bytes = decodeFmMfm(rawbits).slice(0, MICROPOLIS_ENCODED_SECTOR_SIZE);
ByteReader br(bytes);
br.read_8(); /* sync */
int syncByte = br.read_8(); /* sync */
if (syncByte != 0xFF)
return;
_sector->logicalTrack = br.read_8();
_sector->logicalSide = _sector->physicalHead;
_sector->logicalSector = br.read_8();
if (_sector->logicalSector > 15)
return;
if (_sector->logicalTrack > 77)
if (_sector->logicalTrack > 76)
return;
if (_sector->logicalTrack != _sector->physicalCylinder)
return;
br.read(10); /* OS data or padding */